Male 3 month old (adult group) and 24 month old (aged group) C57B1 mice were used in this study. SAN preparations were isolated by micro-dissection and spontaneous pacemaking activity was recorded by measuring extracellular potentials. SANs were identified by anatomic landmarks (crista terminalis, CT), superior vena cava (SVA) and inferior vena cava (IVC). SAN was fixed in a heated tissue bath approximately 37C by temperature controller) on the stage of a microscope and superfused with a solution with or without drugs at a rate of approximately 2 ml/min. The composition of the solution was as follows (in mM): 120 NaCl, 20 NaHCO3, 1 NA2HPO4, 5.4 KCl, 1.8 CaCl2, 1.0 MgCl2, and 5.5 glucose. The solution was bubbled with 95% 02-5%CO2. An insulated/Teflon-coated platinum electrode that was placed in the SAN region was used to record extracellular signals. The beating rate of the SAN was recorded by Neurolog system NL900D and later analyzed with a homemade analysis software. [unreadable] At baseline conditions, average beating rates of SAN preparations are 40613 beat per min (bpm) and 30614 bpm in the adult and aged group, respectively (n=8 in each group, P<0.01). The intrinsic beating rates of SAN show similar difference when 2.5 M propranolol and 10 M atropine are added to the perfusate. When stimulated with beta-adrenergic agonist, Isoproterenol (ISO), both adult and aged SANs show significant differences in pacemaking acceleration and sensitivity (n=4). However, when pacemaking function is inhibited by muscarinic receptors agonist, carbochol (CCh), aged SANs show less sensitivity at all concentrations compared to adult SANs (n=5, P<0.05). Thus, there is a significant difference in pacemaking activity in aged and young mice. SANs from aged mice show decreased intrinsic pacemaking function compared to adult ones. Under extracellular receptor stimulations, drug-sensitivity and pacemaking acceleration/inhibition of aged SANs are significantly different from their young counterparts. This may be due to age-associated remodeling of intracellular signaling and/or pacemaking mechanisms related to aging.